Many processes, industrial and biological, produce waste heat. Waste heat recovery methods previously implemented by various industries have included heat-to-heat technologies that transfer heat from unwanted places to processes or locations where thermal input is needed and heat-to-power technologies that convert heat to energy (e.g., electricity).
Heat-to-heat technologies include heat exchangers and preheaters, and such technologies are commonly implemented in high temperature (e.g., >650° C.) processes, such as melting furnaces and incinerators, to transfer exhausted heat to other fluid streams. As temperature decreases, however, heat transfer kinetics slow, and larger surface areas are required to maintain efficiency. Low temperature heat flows are also often not useful for industrial processes, necessitating an upgrade using additional equipment such as a heat pump. As a result, heat exchanger technology is uncommon for heat streams of <150° C.
At more moderate temperatures (e.g., 250° C. to 650° C.), heat-to-power technologies can be used, such as in systems implementing Rankine power cycles. Steam turbines are common examples of such technology, and typical steam turbines using water as the working fluid can only operate efficiently above about 350° C. Although organic Rankine cycles (using a lower boiling organic material with as the working fluid) can achieve efficiencies of 10-20% below 250° C., specific optimization is typically required, and the organic working fluid raises toxicity, flammability, explosivity, and environmental concerns.
Relatively few options are presently known for recovering low temperature (e.g., <250° C.) waste heat. Examples of known sources of low temperature waste heat (and the typical temperature range of the waste heat) include steam boiler exhaust (150-260° C.), exhaust gas from recovery devices (70-250° C.), process steam condensates (50-90° C.), cooling water return (30-250° C.), and drying/baking ovens (90-250° C.). Computing data centers are further sources of low temperature waste heat, and for example, exhaust air from data racks can produce fluid temperatures of about 50-100° C. above ambient. Although microscale devices (e.g., thin film thermoelectric modules) have shown promise at the microchip level, no effective macroscale solution is presently known. There remains a need in the art for further options for recovery of low temperature waste heat.